This invention relates in general to ditching machines used for digging trenches in which pipelines and cables are buried and, more particularly, to a device for separating rocks from the excavated material.
Ditching machines are widely used to dig extended trenches in which pipelines and other items such as cables are buried. The excavated rock and dirt is typically deposited in a row along one side of the trench and is subsequently used as backfill material once the pipeline has been laid into the trench. During backfilling of the trench, it is important that large objects such as rocks are segregated from the backfill material to prevent damage resulting from such objects falling onto the pipeline. Rocks and similar abrasive items must also be segregated from contact with the buried pipeline to prevent the abrasive action of such objects rubbing against the pipeline during the repeated thermal expansion and contraction experienced by the pipeline.
Padding material in the nature of small aggregates such as sand or crushed caliche is conventionally used to provide a layer of padding surrounding the pipeline to protect it during backfilling operations and from the abrasive action of rocks. Padding material of this type must be continuously transported to the pipeline trench or a large hopper must be provided on the backfilling machine to ensure a constant supply of such material. In either event, transportation of the padding material to the pipeline trench adds significantly to the costs associated with the backfilling operation.
Other types of conventional backfilling machines pick up and process the previously excavated material which has been deposited in a row along the trench to separate the fine material from the coarse aggregates. The fine material is then returned to the trench as padding for the pipeline while the coarse aggregates are replaced in a row alongside the trench. An example of such a machine is disclosed in U.S. Pat. No. 4,633,602. While this type of a machine is desirable because it eliminates the need for transporting padding material from a remote location, its specialized and expensive nature limits its availability for use on many types of projects.